1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus for performing a handoff.
2. Description of the Related Art
In general, for effective management of frequency resources, a mobile communication system divides a radio network into small cell areas, and a mobile station is provided with a service from a base station in a cell to which it belongs. When a mobile station travels from a cell in service to another cell, a handoff is performed for the mobile station to secure a continuous service.
As stated above, a mobile communication system performs a handoff to maintain a call, when a mobile station travels from one cell to another cell. Handoffs are generally divided into soft handoffs and hard handoffs. With regard to the soft handoff, a service is concurrently provided through a channel assigned from an adjacent handoff target base station and a channel assigned by a service base station and then, the channel from the service base station is disconnected when a channel quality of the service base station becomes lower than a predetermined threshold. With regard to the hard handoff, when a channel quality of the service base station becomes lower than a threshold during a call, the channel to the service base station is disconnected and thereafter, a new channel is assigned from an adjacent base station.
FIG. 1 illustrates an overall system structure of a mobile communication system. Referring to FIG. 1, a mobile station (MS) 111 from a plurality of mobile stations belonging to one cell communicate with a corresponding base station (BS). A plurality of base stations 121-12n are controlled by a corresponding base station controller (BSC) 131. A plurality of base station controllers 131-13n are controlled by a mobile station controller (MSC) 141. When a mobile station 111 attempts to call to a wire subscriber 161 in a public switched telephone network (PSTN) 151 through a radio network(BS, BSC, MSC), the mobile switching network 141 establishes a communication channel by connecting the mobile station 111 to the wire subscriber 161 through the PSTN 151.
FIG. 2 illustrates the variation in signal power received from a service base station BS1 and an adjacent base station BS2, when a mobile station travels from a position xe2x80x9caxe2x80x9d to a position xe2x80x9cbxe2x80x9d in a overland area where a service area CELL1 of the service base station BS1 overlaps with a service area CELL2 of the adjacent base station BS2.
A description will now be made regarding a conventional handoff procedure with reference to FIG. 2, on the assumption that a mobile station is presently being provided with a service from base station BS1, and travels to a cell area CELL2 of the adjacent base station BS2.
The soft handoff is performed under the control of a base station controller (not shown) when the mobile station is situated in the overland area where the service area CELL1 of the BS1 and the service area CELL2 of the BS2 overlap each other. That is, the mobile station being presently provided with a service from the BS1 detects the power of a signal from BS2, and if the detected value is determined to be greater than a predetermined threshold value T_ADD, sends the detected value to a base station controller. The base station controller then determines whether BS2 is presently available to assign a channel between the BS2 and the mobile station. When a channel is assigned for BS2, BS1 and BS2 simultaneously provide a service to the mobile station through the respective assigned channels. In the meantime, as the mobile station continuously moves toward BS2 (i.e., from point a to point b), the mobile station eventually reaches a position where the signal power received from BS1 drops below a previously determined drop threshold T_drop for a handoff. Upon reaching a signal from BS1 at the drop threshold, the mobile station drops BS1, presently in service, and is provided with a service only from BS2. At this point, the mobile station does not drop BS1 immediately after the received signal power drops below the threshold T_drop, but only after delay of a guard time T_Tdrop. That is, when the signal power has dropped below the threshold T_drop, the mobile station drops BS1 after a lapse of the guard time T_Tdrop, so as to prevent a handoff from being erroneously performed due to an instantaneous drop of the signal power. However, in this case, the mobile station might erroneously select a farther base station rather than an adjacent base station as a handoff target base station.
This situation will be described in detail with reference to FIG. 3. When a mobile station MS5 moves from a cell serviced by base station BS1 towards a cell serviced by base station BS2, the mobile station MS5 determines a handoff target base station depending on pilot power. Preferably, the mobile station MS5 should determine that the closest base station BS2 as the handoff target base station, in a handoff area. However, in the case where the signal power from BS2 becomes lower due to bad surroundings such as a building, a signal from BS3 which is farther from MS5 than BS2 will transmit a higher power. In this case, MS5 performs a handoff to the far base station BS3 rather than the adjacent base station BS2. That is, when the signal power from the far base station BS3 becomes instantaneously higher than T_add due to surroundings, MS5 accepts BS3 in the activation set and performs a handoff add process. However, before long, the signal power from BS3 will become lower than the signal power from BS2, so that a handoff process will be performed again. Further, an abrupt drop of the signal power may completely disconnect a call.
The conventional handoff has the following problems:
(1) The mobile communication system will experience an increased load due to the unnecessary handoffs described above.
(2) When BS3 is added to the activation set, a communication channel is assigned to MS5, thereby wasting channel resources.
(3) Handoff is performed depending only on the pilot signal power from the base stations, causing handoff errors according to topography.
(4) The traveling direction of the mobile station is not considered for the handoff. Therefore, when the adjacent base station has a low signal power, the mobile station continues to maintain a traffic channel to the present service base station and fails to assign a traffic channel to the adjacent base station, thus causing a call disconnection.
It is, therefore, an object of the present invention to provide a device and method for performing a handoff taking into consideration a traveling direction of a mobile station in a mobile communication system.
It is another object of the present invention to provide a device and method for performing a handoff, in which a mobile station provides its present location and traveling direction to a base station during a handoff request.
It is a further object of the present invention to provide a device and method for performing a handoff depending on information about a location and a traveling direction, provided from a mobile station during a handoff request.
It is still another object of the present invention to provide a device and method for performing a handoff, which restrictively performs a handoff cancel upon receipt of a handoff cancel request from a mobile station.
To achieve the above objects, a device for performing a handoff in a mobile communication system includes a mobile station for transmitting a handoff request message including a measured signal power value and information about a traveling direction thereof, when signal power from an adjacent base station is determined to be higher than a predetermined threshold value; and a base station for performing a handoff depending on the measured signal power value and the traveling direction, upon receipt of the handoff request message.